Abstract
Background:
The initial fixation step is paramount in histology; the eventual quality of a tissue section hinges largely on the specific fixative chosen and how long the sample remains submerged.

Aim:
This study aimed to compare the histological quality of the liver and gills of Nile tilapia (Oreochromis niloticus) fixed in 10% neutral-buffered formalin (NBF) and Bouin’s solution over various immersion times (1, 7, 14, and 21 days).

Methods:
Histological quality, including membrane integrity, staining affinity, and cytoplasmic density, was assessed using hematoxylin-eosin staining and observed under a light microscope at 400x magnification. A qualitative scoring grade (1=poor, 2=fair, 3=good, 4=excellent) was used, and data were presented as median (interquartile range). Statistical analysis was performed using nonparametric Friedman test, Wilcoxon signed-rank test, and Mann–Whitney U test (p < 0.05).

Results:
A 1-day immersion in both NBF and Bouin’s solution yielded the highest histological quality for both organs. In liver tissue, 10% NBF provided superior preservation during the first week of immersion (days 1–7; p = 0.008). However, by Day 21, Bouin’s solution exhibited significantly higher structural stability for the liver than NBF (p = 0.032). No significant differences were observed between NBF and Bouin’s solution in gill tissues until day 14 (p > 0.05). However, a significant divergence occurred on Day 21, when Bouin’s solution outperformed 10% NBF (p = 0.008).

Conclusion:
Under the tested conditions, a 24-h immersion duration provides the most consistent results for Nile tilapia liver and gills. While 10% NBF is effective for short-term liver preservation, Bouin’s solution offers better resilience for both organs during prolonged immersion.

Key words: Hematoxylin–Eosin; Histology; Immersion duration; Teleost; Tissue preservation.